Bonding cords with blocked isocyanates

ABSTRACT

Adhesion of cord fabric, especially polyester cord fabric, to elastomer compounds is achieved using an adhesive made from blocked isocyanates. The blocked isocyanates are dissolved in an organic solvent, and then dispersed into water to form an aqueous mixture. The cord fabric is dipped into the aqueous mixture and heated to dry the cord and dissociate the blocked isocyanate. Cord fabric thus treated is calendered to coat it with an elastomer compound, and the coated fabric is assembled into composite articles of cord and elastomers (such as tires or belts) which are vulcanized with heat and pressure.

United States Patent Georges 1 Feb. 15, 1972 [54] BONDING CORDS WITHBLOCKED 2,976,202 3/196] Salem et al ..l56/1l0 A ISOCYANATES 3,234,0672/l966 Krysiak ....l56/l10 A X 3,503,934 3/1970 Chilvers ..l56/33l X[72] Inventor: Louis W. Georges, Akron, Ohio 73 A Th F- es) '1" & R bbeC Primary Examiner-Leland A. Sebastian 1 sslgnee u r ompany Attorney-S.M. Clark and Gordon B. Seward [22] Filed: Jan. 9, 1970 [57] ABSTRACT[21] Appl. N -I 1,396 Adhesion of cord fabric, especially polyester cordfabric, to elastomer compounds is achieved using an adhesive made [52] U8 Cl 1561308 7 I761. 152/357 from blocked isocyanates. The blockedisocyanates are dis- "156/l10A 56/351 161/66 solved in an organicsolvent, and then dispersed into water to [5] 1 Int Cl 6 5/02 form anaqueous mixture. The cord fabric is dipped into the [58] Fieid /3aqueous mixture and heated to dry the cord and dissociate the ll 1 A 1,161/60 blocked isocyanate. Cord fabric thus treated is calendered tocoat it with an elastomer compound, and the coated fabric is assembledinto composite articles of cord and elastomers [56] References cued(such as tires or belts) which are vulcanized with heat and UNITEDSTATES PATENTS Pressure- 3,460,973 8/ l 969 Hantzer et al 156/1 10 A X 6Claims, No Drawings BACKGROUND OF THE INVENTION This invention relatesto a process for treatment of cord fabric which produces improvedadhesion of the fabric to an elastomeric compound; and to the compositefabric-elastomer produced thereby. More specially, the invention relatesto a cord-dipping process which yields an improved adhesive bond betweenthe cord and the rubbery elastomer which surrounds the cord; and tocord-reinforced elastomeric products thus produced, motor vehicle tires,for example.

In the manufacture of motor vehicle tires, a continuing demand forstronger, more heat-resistant carcasses has promoted the adoption ofincreasingly better performing cord materials. The original cotton cordwas completely replaced by rayon; the development of nylon, and morerecently, polyester cord fabrics, has promoted their wide use.Additionally, metal and glass cord fabrics have captured a portion ofthe tire market.

Adhesion to rubber has always been the first technical barrier to theintroduction of improved cord materials. The problem of adheringpolyester cords to rubber is one which has occupied the attention oftire scientists to a great degree in recent years. The proposedsolutions to date have not been altogether successful.

Early work on polyester-rubber adhesion systems produced a number ofso-called two-dip" processes, which required that the cord fabric bepassed twice through the treatment ovens. Efforts to avoid this costlysystem, and to devise a cord treatment which could be performed onexisting equipment, have been largely unsuccessful.

The basis of much of the technology of adhesion of dissimilar materialslies with the use of an agent which is, in a sense, difunctional. Thatis, it possesses a functionally or characteristic which is compatiblewith each of the dissimilar materials. Early work with cotton cordbrought about the use of a rubber cement dip, which gave a strongmechanical bond to the cotton fibers and adequate adhesion to thesurrounding rubber. The use of smooth synthetic fibers in cordnecessitated development of a chemical bond to the fiber. Resin systems(such as resorcinol-formaldehyde condensates) in combination with newlatex materials (such as those polymers containing vinyl-pyridinemonomer units) gave an adequate solution to this adhesion problem forrayon and nylon cord.

In polyester cord work, adhesive systems containing isocyanate compoundshave produced some evidence ofa chemical bond. Most of the isocyanatecompounds, however, react quite readily with water, and thus their usein aqueous systems is impossible. The use of isocyanates in organicsolvent systems is very effective, however, the resultant difficultiesof solvent recovery and the hazard of fires have made solventdipsunacceptable to most tire producers.

In recent years, so-called blocked" isocyanate compounds have beendeveloped and marketed which allow the use of the isocyanate-basedadhesives in aqueous systems. lsocyanates which contain phenol orcaprolactam groups, for example, retard the reaction of the isocyanateradical with water. At elevated temperatures the compound dissociates,leaving the NCO groups free to form a bond with the polyester. Cord-dipadhesive systems using these blocked isocyanates have been moderatelysuccessful in giving the required adhesion of the polyester cord torubber.

The study of rubber-to-fabric adhesion deals not only with cord-dippingmethods, but also with the rubber compound which is calendered onto thecord fabric. Systems have been developed which incorporate analdehyde-resorcinol condensate resin in the rubber compound, or includethe resin components, for in situ production of the resin. Inclusion offine particle size silica along with the resin in this type of rubbercompound has given very good adhesion results, even with untreated cordfabric. Where polyester cord fabric is concerned, however, somepretreatment of the cord is necessary.

2 SUMMARY OF THE INVENTION It is an object of this invention to providea process for producing composite articles of fabric and elastomerswhich have improved strength and resistance to heat.

It is another object of this invention to provide a process forproducing pneumatic tires having improved adhesion between the cordfabric and the elastomer compound.

It is still another object of this invention to provide a process fortreating cord fiber material to coat it with an adhesive compound whichprovides chemical affinity between the cord fiber and an elastomericcompound applied thereon.

It is a further object of this invention to provide an adhesive processrequiring a considerably lower than usual pickup of adhesive compound.

It is yet another object of this invention to provide tires, belts,footwear and similar articles of improved strength and durability.

These and other objects are accomplished by the methods shown.

The invention lies in the process of producing better adhesion of fabricto elastomers by the use of blocked" isocyanate compounds in aqueousemulsion applied to the fabric cords. The blocked" isocyanate compoundsare first dissolved in an organic solvent system. The resulting solutionis then emulsified or dispersed in water, using standard techniques toshear the mixture; and the fabric is coated by known dipping methods.Oven drying of the dipped fabric removes all volatile materials, andalso dissociates the blocked" isocyanate compound to leave it in areactive condition. The fabric is then calendered to coat it with a thinlayer of elastomer compound; the coated fabric then being assembled intoarticles such as tires, belts, etc., and cured to a finished productwith heat and pressure.

The cord material used may be any of the standard reinforcing materialsused for imparting strength and dimensional stability to compositerubber articles. Cotton, rayon, nylon, glass, and metal cords may beused, however, particular advantage is realized when the process of theinvention is used with polyester fibers, which have recently come intowide use in automobile tires. While stranded cord fabric is contemplatedas the preferred fiber form, random fibers may also be employed toreinforce rubber, with adhesion being improved by the process of theinvention.

The elastomeric materials which may be used to advantage include naturalrubber; polybutadiene; polyisoprene; butyl rubber; copolymers of styreneor acrylonitrile with butadiene, styrene or other conjugated dienemonomers; ethylenepropylene-diene monomer elastomers (EPDM); andchloroprene polymers. Of these materials, natural rubber, polybutadiene,and a butadiene-styrene copolymers have the widest usage in this type ofproduct. The elastomers are compounded with standard rubber compoundswell known in the art, including carbon block and other reinforcingfillers, sulfur, accelerators, zinc oxide, antioxidants, antiozonants,plasticizers, retarders, and tackifiers. In addition, the inclusion inthe elastomer compound of a small amount of resorcinol or phenol,together with para-formaldehyde or hexamethylenetetramine is especiallyrecommended. The presence of a reinforcing silica material is also veryeffective for maximum adhesion.

The isocyanate compounds which are recommended are those blocked"isocyanates which liberate reactive NCO groups on heating them toelevated temperatures, but which will not react with water attemperatures below about C. the base isocyanates (unblocked) have thegeneral formula where R is an aliphatic, aromatic or cycloaliphatichydrocarbon radical of from four to 30 carbon atoms. Typical compoundscorresponding to this formula are:

polymethylene polyphenylisocyanate (PAPI) triphenylmethane-trilisocyanate (TMTI) 2,4-tolylene-diisocyanate(2,4 TDI)2,6-tolylene-diisocyanate(2,6 TDI) bitolylene diisocyanate (TODI)dianisidine diisocyanate (DADI) hexamethylene diisocyanate (HDI)m-phenylene diisocyanate (PDI) l-alkyl-benzene-Z,4-diisocyanate(AB-2,4-DI) l-alkyl-benzene-2,5-diisocyanate (AB-2,5-DI)2,6-dialkyl-benzenel ,4-diisocyanate (DBDI) l-chlorobenzene-2,4-diisocyanate (CDI) dicyclohexylmethane-diisocyanate (CXDI)3,3-dimethoxy diphenyl methane-4,4'-diisocyanate (DDM-l-nitrobenzene-2,4-diisocyanate (NDI) l-alkoxy-benzene-2,4-diisocyanate(ABDI) l-alkylbenzene-2,6-diisocyanate (ADI) m-xylylene-diisocyanatel,3-dimethyl-4,6-bis cyanate hexahydrobenzidine-4,4-diisocyanateethylene-diisocyanate propylene-l,2-diisocyanate cyclohexylene-l,2-diisocyanate 3,3'-dichloro-4,4-biphenylene diisocyanate2,3dimethyl-tetramethylene diisocyanate p,p'-diphenylene diisocyanate2-chlorotrimethylene diisocyanate butane--triisocyanate trimethylenediisocyanate tetramethylene diisocyanate propylenel ,2-diisocyanatebutylenel ,2-diisocyanate ethylidene diisocyanate These isocyanates areblocked" or capped" by reacting them with phenol, caprolactam, or otherknown blocking agents to produce a compound which will not exhibit thenormal reactivity of isocyanates to active hydrogen-containing compoundsat relatively low temperatures, but which will dissociate on heating totemperatures substantially above those at which the dipping compound ismaintained, up to about 250 C.

Since the process of dipping cord fabric to apply an adhesive ispreferably done with water-based adhesives the isocyanate basedcompounds of the invention (suitably blocked") must be incorporated intoan aqueous system. It is recommended that this procedure be done bydissolving the blocked" isocyanate in appropriate organic solvents, andthen emulsifying the solution in water. Suitable solvents are thosewhich will completely dissolve the isocyanate, or, in partiallydissolving it, leave a residue which may be separated by filtration ordecantation. The choice of the appropriate solvent is dictated largelyby the characteristics of the particular isocyanate compound and anyconvenient solvent which will dissolve the blocked isocyanate compoundmay be used. For example, chlorinated aliphatic or aromatic compounds,such as trichloroethylene or chlorobenzene are effective. Dimethylsulfoxide is also a good solvent for the blocked isocyanates, and workswell in the process of the invention.

The solvents may be used alone, or in combinations of two or more, togive the proper characteristics of solubility and boiling point.Solvents which are somewhat water soluble, such as the oxygenatedsolvents mentioned, are especially preferred, since they themselves willnot have to be emulsified. Thus a multiphase system is avoided bychoosing solvents which have at least a limited solubility in water.Since the solvents are volatilized in the drying treatment,consideration should be given to possible hazards of handling vaporsthus produced. Particular solvents which perform well when used with therecommended isocyanates are dioxane, tetrahydrofuran, methylcellosolve,and furfural. Known emulsifying techniques are employed in producing theemulsions, using high-shear agitation. Although an emulsifying agent is(-isocyanateethyl)-benzene-diisonot essential, the use ofcertaincationic or amphoteric emulsifying agents has been shown to bebeneficial. Such materials are:

I-Iyamine 2389 (methyl dodecylbenzyl trimethyl ammonium chloride)Ethomeen C15 ethylene oxide condensation production or primary fattyamines) Triton QSl5 (oxyethylated sodium salt of amphoteric surfactant)Antaron FC34 (amphoteric fatty amino amide) Lauryl pyridinium chlorideAnionic and nonionic emulsifiers, while they are effective in producinga good emulsion, give cord adhesion test results which are not as goodas those obtained with the cationic or amphoteric agents listed.Preferred amounts of emulsifier are from 0.07 to 1.50 percent by weight,based on the total emulston.

Aqueous dip formulations may be effectively used containing from about0.l to about 20 percent of the isocyanate compound, by weight. Theconcentration used is important in determining the final pickup ofadhesive on the cord fabric after drying. The pickup can be relativelylow as compared with conventional latex-resin dips used with rayon andnylon cord treatments, and from about 0.1 percent up to about 10percent, preferably from about 0.5 to 3 percent provides good cordadhesion.

The dipping operation is carried out in conventional continuous-dryingovens designed for cord fabric dipping. These ovens generally employseveral heating zones, and are equipped to apply varying tensions to thecord fabric during its travel through the successive zones. Conventionaldrying temperatures from about to about 250 C. are used, with cordfabric tensions adjusted to first stretch, then relax tension on thecord fabric. Total time required for drying can be varied over a widerange without seriously affecting adhesive properties. Best results arefound with a three-zone oven having a temperature of from 150 to 200 C.in the first zone, and from 200 to 250 C. in the second and third zone.A stretch of from about 2 to 8 percent in the first zone, followed by arelaxation of from I to 3 percent in each of the remaining zones givesoptimum results. Dwell times of from I00 to 250 seconds, 30 to 55seconds, and 20 to 45 seconds in the respective zones produce adequatedrying of the fabric.

Calendering of the elastomer compound skim stock" onto the cord fabricis done in conventional equipment, using stan dard methods. Aspreviously indicated, the preferred elastomer compound is based onnatural or styrene-butadiene copolymer rubber, and is compounded with aresin system along with the conventional compounding ingredients. Fineparticle-size silica may also be included.

Tires or other composite articles are assembled or built in conventionalways, and the articles are vulcanized in standard molds with normaltemperatures and pressures.

A more complete understanding of the invention can be obtained byreference to the following examples of specific embodiments, in whichall percentages are by weight, unless otherwise specified.

SPECIFIC EMBODIMENTS OF THE INVENTION TABLE I Trial l 2 3 4 ExcellentExcellent Trials 2 and 3 were repeated, substituting tetrahydrofuran,and then dioxane for the furfural. Excellent solution was obtained ineach case. (Solutions described as excellent" contained no undissolvedisocyanate; those described as good contained a small amount ofundissolved solids.)

EXAMPLE I] Using the solutions of Example I, dispersions of theisocyanate solutions in water were prepared. The solution from trial 2(48gm.) was added slowly in a fine stream with vigorous agitation to 352gm. of water. A milky dispersion resulted, with the fine particles ofthe isocyanate compound tending to remain in suspension. The procedurewas repeated, using the solutions from the other trails, andincorporating from 0.3 g. to 1.8 g. of various nonionic, cationic,anionic and amphoteric surfactants. The emulsions obtained were allsatisfactory, and appeared slightly more stable than the emulsion withno surfactant.

Using solutions of the isocyanate in dioxane, tetrahydrofuran, andfurfural alone, the emulsification procedure was repeated. However,these solutions yielded dispersions which contained some agglomerates,showing the advantage of the presence of some methylcellosolve in thesolvent system. EXAMPLE III In order to evaluate the effect of varyingthe solvent, the emulsifier and the drying treatment on the adhesion ofpolyester cord to rubber, a number of trials were performed. In eachcase, polyester fibers were dip coated and ovendried; then the coatedcord was calendered to apply a coating of elastomer compound to it; thecoated fabric was vulcanized into a test pad of elastomer, and adhesionvalued were measured.

One of the first standard adhesion tests for tire cord was the H test,described in India Rubber World, Vol. ll4 pp. 213-217 (1946). Variationsof this test, such as the U adhesion test and the T" adhesion test havebeen developed to meet the needs of the industry. All these testsmeasure the force required to pull a single cord out from thesurrounding rubber; some ofthe tests being run at elevated temperaturesto simulate extreme conditions of speed and loading in a tire. In thefollowing trials, T-pullout adhesion was measured at 1 C., and valuesare given as pounds per inch.

To evaluate the effect of the emulsifier, a series of trials was runusing first no emulsifier, then anionic, nonionic, cationic andamphoteric emulsifiers. Cords dipped with the adhesive compounds thusproduced were dried, calendered and tested for adhesion, with thefollowing results as shown in Table II.

The foregoing results demonstrate that adhesion is markedly improved byusing either a cationic or an amphoteric emulsifier, and that noimprovement is realized with the use of anionic or nonionic emulsifiers.

EXAMPLE IV To evaluate the effect of emulsifier level, using thematerial found most effective in Example Ill, a series of trials was runin which the content of lauryl pyridinium chloride was varied from O to1.5 percent by weight based on total emulsion weight. The followingadhesion results were obtained, as summarized in Table III below. Thesolvent used for dissolving the blocked isocyanate (acaprolactam-blocked (PAPl was a 7 to 1 mixture of methyl cellosolve andtetrahydrofuran.

TABLE Ill Percent Lauryl Pyridinium Chloride Adhesion, lh./in.

The foregoing results indicate that adhesion improves with increasingamounts of emulsifier up to about 1.0 percent by weight of the totalemulsion.

EXAMPLE V Tires are built from polyester cord fabric which is treatedwith adhesive compositions corresponding to those which produced highadhesion values in the preceding examples. Tests of these tires underextreme service conditions show a resistance to ply separations andother failures attributable to substandard adhesion of cord to rubber.What I claim:

1. In the process of producing a composite article of fabric andelastomers by the steps of coating cord fabric with an isocyanate-basedadhesive, calendering a coating of compounded elastomer onto the cordfabric, assembling the cord fabric thus treated into a compositestructure together with other elastomeric and metallic elements andvulcanizing the whole with heat and pressure to produce the finishedarticle, the improved process of coating the cord fabric, comprising thesteps of dissolving a blocked isocyanate compound which is a reac' tionproduct of phenol or caprolactam with an isocyanate compound having thefromula where R is an aliphatic, aromatic or cycloaliphatic hydrocarbonradical of from 4 to 30 carbon atoms in a solvent for said blockedisocyanate compound which is dioxane, tetrahydrofuran, methylcellosolve,furfural, or dimethyl sulfoxide,

dispersing the solution thus produced in water to form an aqueousmixture, dipping the cord fabric into the aqueous mixture, and heat ingthe dipped cord fabric, whereby the water is driven off and the blockedisocyanate compound dissociates. 2. The process of claim 1, wherein thesolution is emulsified in water using an emulsifier.

3. The process of claim 2, wherein the emulsifier is cationic oramphoteric.

4. The process of claim 3, wherein the emulsifier pyridinium chloride.

5. The process of claim 1, wherein the cord fabric is a polyester cordfabric.

6. The process of claim 1, wherein the compounded elastomer calenderedonto the cord fabric comprises a mixture of a natural or syntheticrubbery elastomer, a sulfur curing system, finely divided silica,resorcinol, and a formaldehyde donor.

is lauryl

2. The process of claim 1, wherein the solution is emulsified in waterusing an emulsifier.
 3. The process of claim 2, wherein the emulsifieris cationic or amphoteric.
 4. The process of claim 3, wherein theemulsifier is lauryl pyridinium chloride.
 5. The process of claim 1,wherein the cord fabric is a polyester cord fabric.
 6. The process ofclaim 1, wherein the compounded elastomer calendered onto the cordfabric comprises a mixture of a natural or synthetic rUbbery elastomer,a sulfur curing system, finely divided silica, resorcinol, and aformaldehyde donor.